The effects of respiratory acidosis in the chick embryo.

The avian embryo develops within an eggshell which limits the rate of gaseous exhange between the bloodstream and the environment. In the latter half of normal incubation a considerable respiratory disturbance develops so that in the blood of the embryonic fowl the PCOi rises from 20 to 60 mmHg within about 8 days (Dawes & Simkiss, 1969; Freeman & Misson, 1970). A variation of this magnitude in the adult fowl would cause a change in blood pH from 77 to 7-2 in the absence of any compensatory adjustments. In fact the adult fowl appears to be relatively poor in compensating for respiratory acidosis (K. Simkiss, unpublished), and in calculating the figures shown in Table 1 it has been assumed that there is virtually no change in plasma bicarbonate during an acute respiratory acidosis (Hunt & Simkiss, 1967). In contrast to this it has been shown that in the embryo there is a large increase in plasma bicarbonate during normal development so that the change in blood pH is greatly reduced (Dawes & Simkiss, 1969). The situation is shown in Table 1 with the implication that the pH of the blood is less variable in the embryo than it would be in the adult fowl subjected to similar circumstances.